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Transcript
Chapter 19- Viruses Viral Genomes • Viral genomes may consist of Double- or single-stranded DNA Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus. Capsids and Envelopes A capsid is the protein shell that encloses the viral genome. A capsid can have various structures. Viral Envelope: protects the virus and helps the virus attach to the cells that will be infected. Produced in the ER of the infected cell. Virus Structure Nucleic acid enclosed in a capsid (protein coat) and, sometimes, a membranous envelope (proteins derived from host cell). Genetic material can be DNA or RNA, linear or circular, single- or double-stranded. Viral Reproductive Cycles Reproduce only within a host cell (obligate intracellular parasites). Identify host cells by a “lock-and-key” fit between proteins on the outside of the virus and specific receptor molecules on the surface of host cells. In general, the virus injects its genome into the host cell → it is copied and processed to make new viral parts → these parts self-assemble into new viruses that exit the cell (see next slide). simplified viral reproduction cycle Reproduction of Bacteriophages Lytic Cycle – culminates in the death of the host cell Similar to general reproductive cycle of viruses. Phages that reproduce only by the lytic cycle are called virulent phages. Lysogenic Cycle – does not destroy the host cell Viral DNA (prophage) is incorporated into host cell’s chromosome & replicated when the bacterium reproduces. Occasionally a prophage will exit the host chromosome and initiate the lytic cycle to produce new viruses. Phages that reproduce using both types of cycles are called temperate phages. T4 Phage Lytic cycle Temperate Phage Life Cycle Viral Envelopes Many viruses that infect animals have a membranous envelope. Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell (attachment). Reproduction of Enveloped RNA Virus New virus capsids wrap themselves in membrane as they bud from the cell. Provirus A provirus is a virus genome that has integrated itself into the DNA of a host cell. One kind of virus that can become a provirus is a retrovirus. When a retrovirus invades a cell, the RNA of the retrovirus is transcribed into DNA by reverse transcriptase, then inserted into the host genome by an integrase. RNA as Viral Genetic Material The broadest variety of RNA genomes is found in viruses that infect animals. Retroviruses use reverse transcriptase to copy their RNA genome into DNA. HIV is the retrovirus that causes AIDS. The viral DNA that is integrated into the host genome is called a provirus. Unlike a prophage, a provirus remains a permanent resident of the host cell. The host’s RNA polymerase transcribes the proviral DNA into RNA molecules. The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell. LE 18-10 HIV Membrane of white blood cell HOST CELL Reverse transcription Viral RNA 0.25 µm HIV entering a cell RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation New HIV leaving a cell mRNA Provirus Class/Family Envelope Examples/Disease I. Double-stranded DNA (dsDNA) Adenovirus No Respiratory diseases, animal tumors Papovavirus No Papillomavirus (warts, cervical cancer): polyomavirus (animal tumors) Herpesvirus Yes Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma) Poxvirus Yes Smallpox virus, cowpox virus Class/Family Envelope Examples/Disease II. Single-stranded DNA (ssDNA) Parvovirus No B19 parvovirus (mild rash) III. Double-stranded RNA (dsRNA) Reovirus No Rotavirus (diarrhea), Colorado tick fever virus Fifth disease and Rotavirus Class/Family Envelope Examples/Disease IV. Single-stranded RNA (ssRNA); serves as mRNA Picornavirus No Rhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) viruses Coronavirus Yes Severe acute respiratory syndrome (SARS) Flavivirus Yes Yellow fever virus, West Nile virus, hepatitis C virus Togavirus Yes Rubella virus, equine encephalitis viruses Class/Family Envelope Examples/Disease V. ssRNA; template for mRNA synthesis Filovirus Yes Ebola virus (hemorrhagic fever) Orthomyxovirus Yes Influenza virus Paramyxovirus Yes Measles virus; mumps virus Rhabdovirus Yes Rabies virus VI. ssRNA; template for DNA synthesis Retrovirus Yes HIV (AIDS); RNA tumor viruses (leukemia) Measles, ebola, and rabies HIV Bacterial defenses Bacteria protect themselves against phage infections in many ways. They make "restriction endonucleases" which are enzymes that cut the incoming phage DNA into pieces, thus inactivating the infection. They also make a modifying enzyme that protects their own DNA from being attacked by the restriction endonucleases. Viral Diseases in Animals Because viruses typically damage or kill cells, the amount of damage a virus causes to an organism depends partly on the ability of the infected tissue to regenerate by cell division. (ex) Influenza – most people recover fully because the epithelium of the respiratory tract can efficiently repair itself. (ex) Poliovirus – damage to mature nerve cells is permanent because these cells do not divide. Vaccines – harmless variants or derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen. Most antiviral drugs interfere with viral nucleic acid synthesis (ex) Acyclovir – impeded herpesvirus reproduction by inhibiting the viral polymerase that synthesizes viral DNA. (ex) Azidothymidine (AZT) – curbs HIV reproduction by interfering with the synthesis of DNA by reverse transcriptase. More Viruses Retroviruses Herpesvirus Papoviruses Hepatitis B Oncogenes Those genes that are stimulatory for growth and which cause cancer when hyperactive. Mutations in these genes will be dominant. These genes are called oncogenes. The region of the viral genome (DNA in DNA tumorviruses or RNA in RNA-tumor viruses) that can cause a tumor is called an oncogene. This foreign gene can be carried into a cell by the virus and cause the host cell to take on new properties such as immortalization and anchorage-independent growth. Emerging Viruses Appear suddenly (ex) HIV, Ebola virus, West Nile virus, coranovirus (cause of SARS) Processes that contribute to the emergence of viral diseases: Mutation of existing viruses (ex: flu epidemics) Spread of existing viruses from one host species to another Dissemination of a viral disease from a small, isolated population Viral Diseases in Plants More than 2,000 types are known. Common symptoms = bleached or brown spots on leaves & fruits, stunted growth, damaged flowers or roots. Most plant viruses have an RNA genome. Spread by two major routes: Horizontal transmission – plant is infected from an external source of the virus. Vertical transmission – plant inherits viral infection. Viroids & Prions Simplest infectious agents Viroids = circular RNA molecules that infect plants Prions = infectious proteins Appear to cause degenerative brain diseases in animals Most likely transmitted in food (ex: prion-laden beef from cattle with mad cow disease)
